1. Filed of the Invention
The present invention relates to an image forming apparatus such as a copying machine, laser beam printer (LBP), facsimile machine, etc., based on electrophotography or electrostatic recording technology.
2. Description of the Related Art
One of conventional image forming apparatus of this type is a full-color image forming apparatus such as that shown in FIG. 6.
This image forming apparatus comprises: an image forming portion 100 having an image holding element 101 on which an image is formed with a developing powder; a transfer portion 105 for transferring the developed image formed on the image holding element 101 onto a recording material P such as paper; and a fixing portion 109 disposed at a location on the downstream side of the transfer portion 105, for fixing the image that has been developed and transferred onto the recording material.
The image forming portion 100 comprises: an electrifier 102 such as a corona discharging element for uniformly charging the surface of the image holding element 101; a scanning optical system 103 for scanning and exposing the image holding element 101 electrified with a charge by an optical image E with separated colors thereby forming an electrostatic latent image; a developing unit 104 including developing elements 104y, 104m, 104c, 104bk for yellow, magenta, cyan, and black, respectively, for developing the electrostatic latent image.
An image is formed as follows: First, the image holding element 101 is charged by the charging element 102. An optical image E is then projected onto the image holding element 101 for each separated color thereby forming a electrostatic latent image. A proper developing element 104y, . . . of a developing unit 104 is moved to a developing position opposite to the image holding element 101, and the electrostatic latent image formed on the image holding element 101 is developed, thereby forming a developed image with a developing powder such as resin-based toner.
The developed image is transferred onto a recording material P that has been fed to a location between the image holding element 101 and the transfer roller 105a from a recording material cassette 107 via a feeding system along a path represented by broken lines in FIG. 6.
The transfer portion 105 comprises: a recording material holder 105a including a transfer drum; a transfer electrifier 105b: a sticking roller 105g disposed opposite a sticking electrifier 105c, for electrostatically attracting the recording material P onto the recording material holder 105a; an inner electrifier 105d; and an outer electrifier 105e; wherein a recording-material holding sheet 105f made up of a dielectric material is extended across an open area of the cylindrical surface of the transfer drum 105a thereby constructing a cylinder in an integral form.
As the recording material holder 105a rotates, the developed image on the image holding element 101 is transferred part by part onto the recording material P held on the recording-material holding sheet 105f.
A desired number of colored images are transferred in a superimposed fashion onto the recording material P that is held and carried by the recording-material holding sheet 105f, thereby forming a full-color image.
After the completion of the transfer of the desired number of developed images, the recording material P is separated from the recording-material holder 105a by separating means 108, and then carried to a fixing unit 109 by carrying means 150 such as a carrying belt disposed between the transfer portion 105 and the fixing unit 109. In the fixing unit, the unfixed image is heated and thus fixed. The recording material P is then fed out onto a tray 110.
After the completion of the image transfer process, the image holding element 101 and the recording-material holding sheet 105f are cleaned by a cleaner 106 and a transfer cleaner 114, respectively, so as to remove a residual developing powder from their surfaces, thereby preparing the next image forming process.
However, in the case where images are copied on both surfaces of a recording material P, the recording material P is curled to a great extent toward the image surface after each fixing process. This is particularly true when a high-density image is formed on the first surface. This is because when the recording material cools, the surface on which the developing powder is present will contract to a greater extent than will the surface on which no developing powder is present.
After the fixing process, the curling increases with time as the temperature of the recording material lowers. As a result, when a high-density image is formed on a first surface of a recording material, the recording material often separates from the recording-material holder 105a during the process of forming an image on the second surface, and the curled recording material is carried to the fixing unit 109, which causes a problem described below.
When such a recording material P is carried from the recording-material holder 105a to the fixing unit 109, the recording material P on the recording-material carrying means 150 is curled downward, and therefore the recording material P is partially separated from the recording-material carrying means 150. As a result, the carrying force becomes insufficient, and thus the recording material P sometimes blocks its travelling path, which will cause a failure in carrying a recording material.
Even in the case where a recording material P does not block the travelling path, when an end portion of the recording material P comes into contact with a fixing nip 109a formed by a pair of fixing rollers of the fixing unit 109, mechanical resistance may cause the recording material P to slip along the carrying surface of the carrying means 150 and thus cause a failure in entering the recording material into the fixing nib 109a. Under these circumstances, the trailing end portion of the recording material present between the transfer portion 105 and the image holding element 101 fails to keep good synchronization with the image holding element 101, and therefore the image is disturbed.
Furthermore, if a recording material P is excessively curled, the image face sometimes comes in contact with some element disposed at a location above the recording-material carrying means 150 between the transfer portion 105 and the fixing portion 109, which will cause a deformation in an image. In this case, another problem with the carrying of the recording material P may further occur, and thus the image may be disturbed due to the failure of synchronization at the transfer portion.
The above-described problems occur not only in image forming apparatus based on electrophotography or electrostatic recording technology employing an image holding element having photosensitivity, but also in some types of printers such as an ink-jet printer in which an image is formed directly on a recording material itself by depositing ink droplets on it. Such a printer also has the problem of the recording material being curled due to the difference in expansion between the surface with deposited ink and the other surface with no ink. A trouble in carrying a recording material may cause a deformation in an image formed by an image forming portion, and contact with something located above a carrying means may also produce a deformation in the image.